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Revive ‘dead’ sealed lead acid UPS batteries

Posted by aonomus on July 19, 2009

Just to begin, this guide is not a fix-all solution to dead uninterruptable power supply (UPS) batteries, I will describe what the cause of one particular, common failure mode for sealed lead acid batteries is, and the solution.

Commonly in urban areas (or suburbs), power quality is fair to good, with few brownouts, voltage sags, or blackouts, requiring use of the UPS. Usually a UPS connects the battery to its float voltage, typically 13.8V, so that the battery is at its maximum charge, and so that minimum sulfation occurs. The drawback to float charging is that a slight amount of the voltage causes slight electrolysis in the cells, causing them to dry out. Acid remains, giving a full or near-full voltage reading with no load, however as soon as the battery has a real load (ie: the UPS switches over to battery power), little current can be provided by the small surface area of actual active plate area in contact with a dried out cell, and the voltage drops to near zero. Commercial UPS units have scheduled tests which help provide occasional loads and keep overcharging from occurring, and as a result have slightly longer battery life as a result.

Sealed lead acid (SLA) batteries in UPSs have the electrolyte absorbed into either a silica gel or a fiberglass mat depending on construction. The ‘valve’ is a little confusing as it is actually just a small loosely fit rubber cap, and due a plastic plate always being situated above, can push upwards to release any gas due to overcharging, but will get sucked back down once pressure is reduced. Since sealed lead acid batteries recombine the oxygen and hydrogen produced at either plate, it typically never needs to vent, and when venting occurs, recombining results in a negative pressure.

Symptoms of a dried out battery upon initial inspection are:

  1. Low to moderate voltage, 8-12V
  2. Zero current available, not enough to handle a car lightbulb

After openning the battery, you will notice that the caps are dimpled, this is a sign of a dried out cell, as the water is converted to oxygen and hydrogen and vents from the cell, as it recombines after charging, it sucks the cap back down and has a lower pressure than atmospheric.

Tools you will need:

  • Rubber gloves (disposable or dishwashing) – you are recommended to dispose of these, or only use them for battery work as they may be contaminated with lead.
  • Safety goggles
  • Lab coat (or at least a long sleeved shirt and long pants, no sandals for goodness sake!)
  • A thin flathead screwdriver or 2
  • Paper towel
  • A hemostat
  • A flashlight or a bright lamp
  • Superglue or hobby plastic cement
  • A variable power supply, ideally with current limiting

Step 1: Removing the lid

The first step is to remove the lid, this will certainly void your warranty and will show signs of tampering! Stick the screwdriver into the small slot between case and lid and pry gently, work your way around the perimeter of the cell carefully prying to break all the seals.

Step 2: Remove the caps

First, by now you should be wearing gloves, as any acid that may have vented will be in this area, and on the underside of the lid. Wipe any wet spots with a paper towel and place it in the garbage. Inspect the caps, if you see that they are dimpled, this is a sign that they are dry internally, although not all cells that dry out are like this. Take the hemostats and gently grip the sides, you may hear a sucking sound as the pressure equalizes. Place the cap aside, preferrably where they won’t get lost.

Step 3: Inspect the cells

Take your flashlight or lamp and aim the light into the holes, you should see a white material which looks fairly dry. If it appears shiny and wet, then your cell isn’t dried out and you probably shouldn’t continue. If you see lumps of white material and your battery rattles, this is sulfation and can be dealt with after rehydration.

Step 4: Add water

Take a syringe and needle (preferrably with the tip cut or filed off), and draw 10cc of water. If your battery is over 7Ah, add at least 5, then slowly add the rest, watching with your light to see when the glass matt slows in absorbing water. If after 10cc of water your cell still hasn’t filled, continue to add slowly, and keep track of how much water has been added. After the water stops being absorbed and gives the mat a wet sheen, stop, reset, and fill the other 5 cells (a 12V battery will have 6 cells, 2V each, and 6v batteries have 3 cells). Replace the caps and place the lid on loosely, connect a voltmeter and your power supply set to about 13.5-13.8V.

Step 5: Charge

Set your power supply to the battery with current limit set low, and slowly open it up until voltage is steady. At this point, your battery will be drawing all of zero mA. Give it time, the battery acid is still mixing with the water and distributing itself in the mat. The charging voltage will slowly rise, eventually it will reach a nice current (on a 7Ah battery I had a peak of 600-700mA constant current), and slowly taper off. Increase the current limit as necessary to allow the battery to charge, try not to exceed 1/10C, as you want a gentle charge. At some point for large batteries the current may increase, this is to be expected, try to allow full charging current so long as the battery does not become warm.

During this stage is when you should be monitoring for temperature increase, any hissing, bubbling, or the occasional ‘pop’ (a cell venting). If you hear any of these, reduce the voltage, although slightly increased temperature is to be expected (just slightly above cool metal, something like body temperature, anything warm or hot is not good, reduce charging current).

I have noticed that sometimes a battery will inexplicably decide to self-discharge right down to zero volts and refuse to hold any charge. My guess is that in at least one cell the plates have shorted together, as the battery current draw dramatically increases and the battery begins heating very rapidly. If you do have a battery like this, discard it.

You may also notice that badly sulfated cells cause the battery to charge up to a certain point (typically 14V – 2V*X, where X is the number of badly sulfated cells) and begin warming up. If this is the case, you will probably need to build a desulfator (pulses 50V at 1kHz into the battery) and maintain using a trickle charger (wall wart at maybe 500mA)

Step 6: Seal

At this point once the battery has been charged, and you have tested to see if it will provide current, you can seal the lid back ontop (after placing the caps back on the cells), place some superglue or plastic cement on the areas where the seals were broken. Do not seal the entire perimiter of the lid as it must have gaps to allow for venting of gasses.

This is a ‘living’ article, it will change as I continue to experiment and revive more batteries. I will also at some point post about the desulfator I made.

4 Responses to “Revive ‘dead’ sealed lead acid UPS batteries”

  1. […] Comments (RSS) « Revive ‘dead’ sealed lead acid UPS batteries […]

  2. Larry Baxter said

    Thanx for info. Will try technique. Tired of wasting money on replacing these batts.

  3. peter said

    I do this all the time to give 6 more months of life to SLAs that are on the brink .
    but like you said its a short term fix .

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